When is ALTERNATE MODE for FLAPS used on the 777 ?

[guy 24]

Hi all,

 

The flaps/Slats on the 777 are operated normally by Central HYDRaulic System.(primary mode)

 

If I understand well, When HYDR SYST C is off one has two options:

 

-Use SECONDARY Mode by simply moving the FLAP LEVER (as if all was normal)

  Flaps are then moved by ELECTRICAL motors and SPLIT flaps appear on the Eicas,

  Flaps extension will be very slow, but one can extend them up to 30°

 

-Use ALTERNATE Mode by arming the corresponding Button and setting EXT,

 Flaps are then moved by the SAME (?) ELECTRical   motors and SPLIT flaps appear

 also on Eicas. Flaps extension will also be very slow, but one is limited to 20° and

 has no Asymetry and Uncommanded motion protection,

 

So My questions is: As both modes use the SAME electrical motors and as the

Secondary Mode offers more advantages than the Alternate mode, in what case

would one USE (or be  obliged to use) the ALTERNATE Flaps ?

 

Or otherwise: Which is the FAILURE where SECONDARY mode does NOT work any more

and ALTERNATE Mode still works ?

 

Thanks in advance.

 

Guy

[downscc 1,327]

Guy, full names in the PMDG forums please.

 

Good place to start this discussion is FCOM pg 9.20.4, which schematically indicates the power source (elect or hydra) for each actuator. There are 14 and they have multiple sources, not just central hydra for example. Then on pg 9.20.20 is basically says the central hydra is the normal source of power and the chapter on hydraulics (13) assigns flaps to center too.  Not sure the schematic is wrong, maybe an expert will chime in.

 

Use ALT mode when QRH recommends it.  The only case I found on pg 9.8 is when the flap/slat electronics has failed, which makes sense.

[guy 24]

Hi,

 

Thanks for the answer;

I had read everything about flaps/slats in the FCOM but I hadn't consulted the QRH.

 

So the answer to my question must be that the Flaps SECONDARY mode needs

the FSEU (Flaps/Slats Electronic Unit) while the ALTERNATE Mode does NOT need it  

and bypasses it in some way !

 

Guy

[Jet Tech 31]

Guy,

 

Secondary mode is the preferred mode since it still offers asymmetry and skew protection that is not available in the Alternate mode.   Alternate mode is used if the FSEU is failed and uses limit switches on the flap pdu to set flaps 20 but offers no protection from a Flap Skew, Uncommanded Motion scenario or Flap asymmetry issue.

[guy 24]

Paul,

 

Thanks for the answer.

 

"Alternate mode is used if the FSEU is failed and uses limit switches on the flap pdu to set flaps 20"

 

Had to "google" it : PDu = Power drive Unit.

So that confirms that SECondary  needs the FSEU while ALTernate does not.

 

Guy 

[Jet Tech 31]

That is correct.  So if you want to do this in the PMDG 777,  set up a duel FSEU failure in the PMDG Failures menu and you will be forced to use Alternate mode to extend or retract the Flaps/Slats.   They will be extremely slow so allow plenty of time.

[guy 24]

"They will be extremely slow so allow plenty of time"

I have done the test;

 

Flaps 0-20 Extension  Primary mode :  35 seconds

                                   Secundary Mode : 210 seconds (with full slats)

                                   Alternate Mode :   150 seconds (with slats mid-range)

 

Thanks again for the answers:

 

Guy